Due to its versatility and quantitative nature, nuclear magnetic resonance (NMR) spectroscopy is one of the most commonly used tools in analytical chemistry.1,2 1D 1H NMR experiments are widely applied for the extraction of quantitative concentrations of individual chemical species in solution provided that the spectra are well-resolved. A major advantage of 1D 1H spectra is that the integral of a given peak is directly proportional to the concentration of the compound it belongs to.3 1D 13C{1H} NMR spectroscopy at natural 13C abundance can also be used for quantification by targeted profiling using database information.4 
In the presence of strong peak overlaps, which are typical for complex mixtures such as ones encountered in metabolomics, 1D 1H NMR experiments become less useful, and spectra are difficult to analyze. Significant peak overlaps in 1D NMR spectra of metabolomics samples prevents straightforward quantification through 1D peak integrals. While the resolution issue can often be addressed by 2D NMR spectroscopy, the quantification of 2D spectra is hindered by the variability of cross-peak intensities due to uneven magnetization transfer during the preparation, evolution, or mixing periods because of differences in scalar J-couplings and spin relaxation.5 This feature prevents the direct use of cross-peak integrals as quantitative measures of sample concentrations.
Therefore, what is needed are better methods, including improved NMR methods, that allow the analysis of complex mixtures that are found in metabolomics. In particular, better NMR techniques for the quantitative analysis of mixtures of compounds are needed, such as approaches that might aim to translate cross-peak integrals into concentrations.